The present invention is an improved safety mechanism for geared and gearless traction elevators, which can effectively prevent runaway motion of the car in both the up and down directions, and which can also prevent any unwanted car movement at a landing.
A traction elevator has a car supported by a plurality of ropes, which pass over a drive sheave at the top of the elevator shaft and are connected to a counterweight. Long ago, the elevator car safety was developed to prevent elevator cars, in the event of a rope breakage or other mishap, from falling down the elevator shaft. Typically, the car safety is activated by a governor driven by a cable attached to the car.
The hoist machine, located at the top of the elevator shaft, has a motor for driving the drive sheave to move the car up and down, and a main friction brake to hold the car while parked at landings, when the motor is off. The friction brake is needed because the weight on opposite sides of the drive sheave is usually not equal. The friction brake, which is typically spring-applied and electrically released, is designed to hold any unbalance, ranging from that of an empty car on a high floor to that of a car on a low floor with a 25% overload.
There are several conditions under which the friction brake can fail, however. The brake spring compression may have been misadjusted to produce a "soft" stop in normal operation. The brake linings may have become worn, which will reduce the spring pressure. The brake linings may become contaminated with oil, thereby reducing the coefficient of friction. Or, the brake-release solenoid or other parts could jam or otherwise fail to let the brake apply.
If the brake should fail at a landing, and the car begins to move, the releveling circuit should actuate the motor to keep the car relatively close to the landing. However, there are conditions under which the motor control can malfunction or not be actuated (e.g. a safety shutdown or power failure). Moreover, the motor could disengage from the drive sheave, as a consequence of a broken worm or pinion shaft or broken gear teeth (in the case of a geared elevator).
In the event of a failure while the doors are closed, unwanted car movement in the down direction generally presents only limited consequences, due to the presence of the traditional car safety. If the movement commences sufficiently close to the bottom of the shaft, and the car reaches the car buffer before tripping the governor, the buffer will decelerate the car at a rate less than one "g". If the car reaches the trip speed of the governor, then the car safety will stop the car, and again the deceleration provided by the car safety will be less than one "g".
The typical elevator counterweight is designed to balance the weight of the car plus about 40% of the rated car capacity. In practice, at least 75% of elevator trips are made with less than 40% of rated capacity on board. This means that, in the event of a failure, the car will more often move in the up direction, due to the counterweight side being heavier than the car side.
The existing Elevator Code (Safety Code For Elevators and Escalators) prohibits setting the car safety in the up direction. A small percentage of elevators have counterweight safeties, but for the majority of elevators, if runaway upward travel should occur, the car will continue to accelerate until the counterweight eventually strikes its buffer, possible at a speed far in excess of the rating of the buffer. But, no matter how quickly the downwardly-moving counterweight is stopped, the car will keep going, decelerating only due to gravity, i.e., at one "g". If the overhead clearance is insufficient for the car to stop due to the deceleration of gravity, the car will strike the slab or other obstruction at the top of the hoistway, causing damage and possible injury.
When the car is at a landing with the doors open, any motion of the car, except for a releveling operation, is unintended. Yet, in the event of a failure as described above, even if the car has both a car safety and a counterweight safety, there is nothing to arrest car movement, either up or down, until overspeed conditions are reached or the buffer is hit.